Recently, high order modulation schemes, for example, 16QAM, 64QAM or even 256QAM, have been introduced to achieve high data rate wireless communications. In an actual propagation channel, multipath interference (MPI) caused by multipath fading may severely degrade the performance of wireless communications systems with high order modulations.
Multipath interference cancellation (MPIC) techniques have been proposed as a possible solution to mitigate severe multipath interference. The basic concept of MPIC is to duplicate the interference and remove the interference replica from the received signal. FIG. 1 shows schematically the general structure of a MPIC device.
The MPIC includes a feedback loop including an interference removal unit 10, a demodulator 20 and an interference duplication unit 30. A received signal subject to multipath interference is received from the left of FIG. 1, and an estimate of that signal without multipath interference is output to the right of FIG. 1.
Generally, the MPIC consists of two steps: firstly, based on the output of the demodulator 20 and channel estimates from a channel estimation unit (not shown), the interference duplication unit 30 estimates interference contributed by different paths; secondly, the interference removal unit 10 subtracts the estimated interference from the received signal [1], [2], [3].
Every path will cause interference to each other path. Assuming a system which can resolve m multipath signals, in total (m, 2) interference items should be taken into consideration. Here (m, 2) represents the number of ways of taking 2 items from m items. The calculation of each interference item is almost identical; therefore, the complexity of MPIC is proportional to the number of interference items.
The complexity of MPIC is quite high. For code division multiple access (CDMA) systems, the complexity of MPIC is at least 3 times larger than that of a conventional RAKE receiver [4].